1. Field of the Invention
The present invention relates to ram air parachutes and more particularly to ram air parachutes having an improved canopy design.
2. Discussion of Related Art
Parachutes have evolved over the years into highly sophisticated systems, and often include features that improve the safety, maneuverability, and overall reliability of the parachutes. Initially, parachutes included a round canopy. A skydiver was connected via a harness container system to the canopy by suspension lines disposed around the periphery of the canopy. Such parachutes severely lacked control. The user was driven about by winds with little mechanism for altering direction. Furthermore, such parachutes had a single descent rate based upon the size of the canopy and the weight of the parachutist. They could not generate lift and slowed descent only by providing drag.
In the mid-1960's the parasol canopy was invented. Since then, variations of the parasol canopy have replaced round canopies for most applications, particularly for aeronautics and the sport industry. The parasol canopy, also known as a ram air canopy, is formed of two layers of material—a top skin and a bottom skin. The skins may have different shapes but are commonly rectangular or elliptical. The two layers are separated by vertical ribs to form cells. The top and bottom skins are separated at the lower front of the canopy to form inlets. During descent, air enters the cells of the canopy through the inlets. The vertical ribs are shaped to maintain the canopy in the form of an airfoil when filled with air. Suspension lines are attached along at least some of the ribs to maintain the structure and the orientation of the canopy relative to the pilot. The canopy of the ram air parachute functions as a wing to provide lift and forward motion. Guidelines operated by the user allow deformation of the canopy to control direction and speed. Ram air parachutes have a high degree of maneuverability.
Canopies are flexible and stretchable membrane structures, they distort based upon mechanical and aerodynamic tensions, stresses, airflows and pressure distribution. Although a cell is modeled as having a basically rectangular cross section, when inflated the shape distorts towards round with complex distortions. Under canopies of conventional design, the leading edge or nose of the ram air parachute is deformed during flight as is the top profile of the airfoil between the ribs. Additionally, with forward motion, the head-on wind overcomes the internal pressurization of the canopy, and deforms the nose of the canopy. This distortion blunts the nose of the airfoil or even indents it, impairing the aerodynamics of the parachute wing. The parachute flies less efficiently as a result. Therefore, a need exists for a ram air parachute canopy which reduces nose distortion and spanwise topskin distortion.
The shape of the canopy of a ram air parachute during flight is affected by the air passing through and around the canopy. Under canopies of conventional design, the leading edge or nose of the ram air parachute is deformed during flight. Since the skins and ribs are formed of highly flexible materials, they provide little structure for maintaining the shape of the canopy. The shape is provided by the internal pressurization caused by air entering the inlets.
Typically, in a ram air parachute, suspension lines are attached to every other rib, thus creating loaded ribs (i.e., ribs to which suspension lines are attached) and non-loaded ribs (i.e., ribs which do not have suspension lines attached thereto). The different stresses on the loaded and non-loaded ribs also distorts the cell shape. FIG. 1 illustrates a cross section of a portion of a typical ram air parachute canopy 500 during flight. FIG. 1 shows four cells 501, 502, 503, 504 with three loaded ribs 510, 511, 512 and two non-loaded ribs 521, 522. Suspension lines 541, 542, 543 are attached to the loaded ribs 510, 511, 512. The top skin 530 and bottom skin 531 tend to arc between the ribs during inflation. Also, the non-loaded ribs 521, 522 tend to be higher than the loaded ribs 510, 511, 512, which provides a distortion along the span of the canopy. The distortion is aerodynamically undesirable and reduces the efficiency and performance of the canopy.
In order to keep the loaded and non-loaded ribs level and to improve upon the aerodynamics of the canopy, cross-bracing between ribs has been added to some canopy designs. Cross bracing is the use of diagonal ribs in addition to vertical ribs to create more loaded rib--top skin junctions without adding more lines which increases drag and possible deployment malfunctions. Perfection of the top profile of the airfoil is far more important aerodynamically than the bottom profile. U.S. Pat. No. 4,930,927 illustrates such a design. Cross-braced designs suffer from a number of drawbacks. Cross-bracing results in very complicated construction, high manufacturing costs, and increased packing volume. The standard cross braced design is a ‘tri cell’ construction with a packing volume approximately twenty-five (25)% larger than an equivalent non-cross braced design. Furthermore, the increased rigidness induced by the cross-bracing creates higher opening forces for the pilot. Typically, large cross porting is used on all of the cells to reduce pack volume, which does no thing to slow the canopy's inflation on deployment. The opening the top skin 21a, 21b and bottom skin 22a, 22b. Suspension lines 51a, 51b are attached to the loaded ribs. As illustrated in FIG. 4, the loaded ribs 31, 33 are of the same height. The unloaded rib 32 is shorter in height than the loaded ribs 31, 33. The bottom skin 22 is not flat, but is angled between the loaded ribs 31, 33 and the non-loaded rib 32. The cell has a trapezoidal shape rather than the rectangular shape of conventional cells. During flight, a portion of the force applied from the suspension lines 51a, 51b to the loaded ribs 31, 32 is transferred via the now angled bottom skin portions 22a, 22b to the non-loaded rib 32. The load transfer results in an improved load distribution and reduced span-wise distortion of the top skin.
Sliders used to counteract the large opening forces on a cross-braced canopy often cause premature wear on the suspension lines of the canopy. A slider is a rectangular piece of material with a grommet at each corner. Grouped suspension lines pass through each grommet. When the parachute opens, the force of the opening canopy and separating suspension lines forces the slider down the suspension lines. Air resistance tends to slow movement of the slider and, hence, restrict opening of the canopy against the spreading force of the inflating canopy pushing the slider down. The most force on the slider comes from the lines to the outermost cells, which pushes the slider down rapidly caused friction heat. The heat changes the dimension of many standard types of lines (e.g., Spectra, dyneema brand lines). It is not uncommon for outer lines to change in dimension as much as five inches in only a couple of hundred jumps. Accordingly, cross braced canopies are almost exclusively supplied with Aramid based lines (e.g., Kevlar, Vectran, etc.). These lines do not change dimension with the generated slider-friction heat solving the problem stated above, but suffer from micro-fiber cracking. Accordingly, if over jumped, Aramid lines can break catastrophically with no warning.
Accordingly, a need exists for a parachute design which reduces the top skin distortion of a canopy without using cross braces.